1. Field of the Invention
The present invention relates to optical pickups for recording information to optical disk and reproducing recorded information, and in particular to technology for reducing the size of optical pickups.
2. Related Art
In recent years, optical disk devices applied in wide ranging fields have been used to record information to optical disk such as CD (compact disk) and DVD (digital versatile disk), and to reproduce recorded information.
An optical pickup includes a semiconductor laser diode, an objective lens, and a photodiode. By controlling the orientation of the objective lens to follow the displacement of the optical disk, the optical pickup focuses the laser beam emitted from the semiconductor laser diode at a predetermined position on the recording surface of the optical disk and directs reflected light toward the photodiode (e.g. see Japanese Published Patent Application NO. 10-177733).
By adopting this structure in which the semiconductor laser diode and photodiode are fixed with only the objective lens following the displacement of the optical disk, the number of optical components is increased due to the provision of optical elements in the moving part that follows the optical disk and the fixed part that does not follow the optical disk. This is counter to demands for optical pickup miniaturization. Also, coma aberration is caused by the optical axis of the objective lens not being aligned with the chief ray of the beam due to the displacement of the objective lens.
In regard to this problem, Japanese Published Patent Application NO. 5-109106 discloses a structure in which all of the optical elements are mounted within a single housing, which is made to follow the optical disk as one (hereinafter, this structure is referred to as an “integral optical system”). Miniaturization is possible with an integral optical system type optical pickup due to the reduced number of parts.
FIG. 1 is a cross-sectional view showing a typical structure of this type of optical pickup. As shown in FIG. 1, optical pickup 1000 includes a semiconductor laser diode 1001, reflecting surfaces 1002 and 1003, an objective lens 1004, a photodetector 1006, and a drive coil 1020. Reflecting surface 1002 is formed as a reflective diffraction grating that reflects the beam from semiconductor laser diode 1001. Reflecting surface 1003 is formed as a total internal reflection surface that reflects the beam from reflecting surface 1002. Objective lens 1004 focuses the reflected beam from reflecting surface 1003 on a recording surface 1005. Photodetector 1006 detects the intensity distribution of the beam reflected by recording surface 1005.
With this structure, the beam emitted from semiconductor laser diode 1001 spreads out as it is reflected by reflecting surfaces 1002 and 1003, before being focused on recording surface 1005 by objective lens 1004.
The beam reflected by recording surface 1005 again passes through objective lens 1004 and is reflected by reflecting surfaces 1003 and 1002 while being focused at the same time. First-order diffracted light of the reflected beam diffracted by the reflective diffraction grating of reflecting surface 1002 enters photodetector 1006.
The intensity distribution of the reflected beam from recording surface 1005 differs depending on whether a pit is formed at a focusing position 81a. Photodetector 1006 outputs a focus error signal and a tracking error signal according to the intensity distribution. When these error signals are energized by drive coil 1020, the orientation of optical pickup 1000 is controlled using electromagnetic force to focus the beam at focusing position 81a. Information recorded on recording surface 1005 is read using the output signal of photodetector 1006.
To read pit information on the optical disk precisely with an optical pickup, a part of the emission beam, near the chief ray, having a flat intensity distribution is focused on the optical disk. Consequently, optical properties improve with decreases in the NAO (numerical aperture on object side), directly after emission by the semiconductor laser diode, of the beam that enters the objective lens. Also, NA/NAO (magnifying power) increases with decreases in NAO, given that NA on the optical disk side is stipulated as 0.45 for CD, 0.6 for DVD, and 0.85 for Blu-ray disk.
NAO on the semiconductor laser diode side is the sine of the half-cone angle of the beam that enters the entrance pupil of the objective lens, and is roughly determined by the ratio of the entrance pupil radius of the objective lens to the optical path length from the semiconductor laser diode to the objective lens. Consequently, magnifying power increases with increases in the optical path length from the semiconductor laser diode to the objective lens for a given entrance pupil radius.
Using reflecting surfaces 1002 and 1003 in optical pickup 1000 enables the distance between the semiconductor laser diode and the objective lens to be shortened to one third of that when reflecting surfaces are not employed, with optical path length remaining unchanged. In other words, this structure allows the size of the optical pickup to be reduced in the optical axis direction of the objective lens with magnifying power remaining unchanged.
Furthermore, the fixed positional relationship between objective lens 1004 and the other optical elements in optical pickup 1000 also makes it possible to eliminate coma aberration.